1. Field of the Invention
The present invention relates to active pixel sensors and active pixel sensor arrays. More particularly, the present invention relates to arrays of active pixel sensors wherein each of the active pixel sensors is a triple-junction structure to ensure that each pixel sensor in the array measures each of the three primary colors (R-G-B) in the same location. The active pixel array employs the triple-junction structure of the active pixel sensor to achieve color separation due to the differences in absorption length in silicon of light of different wavelengths. The active pixel array further employs storage nodes to hold a signal after an exposure time has finished and throughout a readout process.
2. The Background Art
Semiconductor devices for measuring the color of light are known in the non-imaging art. These devices have been built with a variety of technologies that depend upon the variation of photon absorption depth with wavelength. Examples are disclosed in U.S. Pat. No. 4,011,016, entitled xe2x80x9cSemiconductor Radiation Wavelength Detectorxe2x80x9d and U.S. Pat. No. 4,309,604, entitled xe2x80x9cApparatus for Sensing the Wavelength and Intensity of Light.xe2x80x9d Neither patent discloses either a structure for a three-color integrated circuit color sensor or an imaging array.
In the imaging art, CCD devices with multiple buried channels for accumulating and shifting photocharges are known. These devices are difficult and expensive to manufacture and have not been practical for three-color applications. U.S. Pat. No. 4,613,895, entitled xe2x80x9cColor Responsive Imaging Device Employing Wavelength Dependent Semiconductor Optical Absorptionxe2x80x9d discloses an example of such a device. This category also includes devices that use layers of thin-film photosensitive materials applied on top of an imager integrated circuit. Examples of this technology are disclosed in U.S. Pat. No. 4,677,289, titled xe2x80x9cColor Sensorxe2x80x9d and U.S. Pat. No. 4,651,001, entitled xe2x80x9cVisible/Infrared Imaging Device with Stacked Cell Structure.xe2x80x9d These structures are also difficult and expensive to make, and have not become practical.
Also known in the imaging art are color imaging integrated circuits that use a color filter mosaic to select different wavelength bands at different photosensor locations. U.S. Pat. No. 3,971,065, entitled xe2x80x9cColor Imaging Arrayxe2x80x9d, discloses an example of this technology. As discussed by Parulski et al., xe2x80x9cEnabling Technologies for Family of Digital Camerasxe2x80x9d, 156/SPIE Vol. 2654, 1996, one pixel mosaic pattern commonly utilized in digital cameras is the Bayer color filter array (CFA) pattern.
Shown in FIG. 1, the Bayer CFA has 50% green pixels arranged in a checkerboard. Alternating lines of red and blue pixels are used to fill in the remainder of the pattern. As shown in FIG. 2, the Bayer CFA pattern results in a diamond-shaped Nyquist domain for green and smaller, rectangular-shaped Nyquist domains for red and blue. The human eye is more sensitive to high spatial frequencies in luminance than in chrominance, and luminance is composed primarily of green light. Therefore, since the Bayer CFA provides the same Nyquist frequency for the horizontal and vertical spatial frequencies as a monochrome imager, it improves the perceived sharpness of the digital image.
These mosaic approaches are known in the art to be associated with severe color aliasing problems due to the facts that the sensors are small compared to their spacing, so that they locally sample the image signal, and that the sensors for the different colors are in different locations, so that the samples do not align between colors. Image frequency components outside of the Nyquist domain are aliased into the sampled image with little attenuation and with little correlation between the colors.
As pointed out above in the discussion of CCD color imaging arrays, the semiconductor processes employed in manufacturing the arrays can be both difficult and expensive to implement. There are, however, CMOS technologies that are known which may be implemented with less expense and greater ease.
Referring to FIG. 3, many modern CMOS integrated circuit fabrication processes use a xe2x80x9ctwin-wellxe2x80x9d or xe2x80x9ctwin-tubxe2x80x9d structure in which a P well region 10 and a N well region 12 of doping density approximately 1017 atoms/cm3 are used as regions within which to make N-channel and P-channel transistors, respectively. The substrate material 14 is typically a more lightly doped P-type silicon (1015 atoms/cm3), so the P well 10 is not isolated from the substrate 14. The N-channel FET 16 formed in the P well 10 includes N+normal source/drain diffusions 18 at a dopant concentration of greater than 1018 atoms/cm3 and N-type shallow lightly doped diffusion (LDD) regions 20 at a concentration of approximately 1018 atoms/cm3. The P-channel FET 22 formed in the N well region 12 is similarly constructed using normal P+source/drain regions 24 and shallow LDD regions 26 of similar dopant concentrations.
Referring to FIG. 4, in an improved process, known as xe2x80x9ctriple wellxe2x80x9d, an additional deep N isolation well 28 is used to provide junction isolation of the. P well 10 from the P substrate 14. The dopant density of the N isolation well 28 (1016 atoms/cm3) lies between the dopant densities of P substrate 14 and P well 10 (1015 atoms/cm3 and 1017 atoms/cm3, respectively). U.S. Pat. No. 5,397,734, entitled xe2x80x9cMethod of Fabricating a Semiconductor Device Having a Triple Well Structurexe2x80x9d, discloses an example of triple well technology.
Triple well processes are becoming popular and economical for manufacturing MOS memory (DRAM) devices, since they provide effective isolation of dynamic charge storage nodes from stray minority carriers that may be diffusing through the substrate.
Storage pixel sensors are also known in the art. In a storage pixel, data representing intensity of light received by a phototransducer are stored in a storage element that can be read out and cleared using appropriate control circuitry.
Accordingly, it is an object of the present invention to provide a color imaging array in which three color bands are sensed with detectors each in the same location, with sensitive areas that are not very small compared to their spacing, such that aliased image components are attenuated, and such that the color samples are aligned between colors.
It is a further object of the present invention to provide an active pixel color imaging array that can be fabricated using a standard modern CMOS memory process.
It is another object of the present invention to provide an array of active pixel sensors having multiple storage nodes for capturing a color image.
The present invention is directed to color separation in an active pixel MOS imaging array utilizing a triple-junction pixel cell structure to take advantage of the differences in absorption length in silicon of light of different wavelengths to measure different colors in the same location with sensitive areas almost as large as their spacing.
In the present invention a color photosensor structure that separates blue, green and red light is formed in a P-type silicon body. The color photosensor structure comprises a vertical PNPN device that implements a triple stacked photodiode and includes a first N-doped region formed in the P-type silicon body, a P-doped region formed in the first N-doped region, and a second N-doped region formed in the P-doped region. A triple well process is employed according to the present invention to fabricate the color photosenor structure. The normal N well of the triple well CMOS process is not employed in the color photosenor structure of the present invention, although it may be useful to use it on the same chip, outside of the array of imager cells.
In the color photosensor structure, the pn junction formed between the P-type silicon body and the first N-doped region defines a red-sensitive photodiode at a depth in the silicon approximately equal to the absorption length of red light in silicon, the pn junction formed between the first N-doped region and the P-doped region defines a green-sensitive photodiode at a depth in the silicon approximately equal to the absorption length of green light in silicon, and the pn junction formed between the P-doped region and the second N-doped region defines a blue-sensitive photodiode at a depth in the silicon approximately equal to the absorption length of blue light. Sensing circuitry is connected to the red, green and blue photodiodes to integrate and store respective photodiode currents.
The present invention reduces color aliasing artifacts by ensuring that all pixels in an imaging array measure red, green and blue color response in the same place in the pixel structure. Color filtration takes place by making use of the differences in absorption length in silicon of the red, green and blue light.
The present invention provides advantages in addition to reduction of color aliasing. For example, it eliminates the complex polymer color filter array process steps common in the prior art. Instead, a triple-well process, which is commonly available in the semiconductor industry is used. Also, overall efficiency of use for available photons is increased. With the traditional approach, photons not being passed by the filter material are absorbed in the filter and wasted. With the approach of the present invention, the photons are separated by absorption depth, but are all collected and used. This can result in an overall improvement in quantum efficiently by around a factor of three.
The present invention provides an excellent example of an imager that would be difficult to implement with conventional CCD technology. In addition, the present invention benefits from the availability of scaled CMOS processing, in the sense that there are many support transistors in each three-color pixel.
According to another aspect of the present invention, the triple-diode sensors of the present invention are employed in active pixel sensors having multiple storage nodes suitable for use in an array of storage pixel sensors.
In a first embodiment of an active pixel according to the present invention, the active pixel sensor includes a plurality of storage nodes. Each storage node stores a pixel of one color of the image. One row select line is provided for selecting the plurality of storage nodes, and a plurality of column output lines are provided upon which the images stored in the plurality of storage nodes may be read out. Column circuits may be employed to perform a function, such as matrixing for color space conversion, on the images stored on the plurality of storage nodes.
In a second embodiment of an active pixel according to the present invention, the active pixel sensor includes a plurality of storage nodes, a plurality of row select lines connected to the plurality of storage nodes and a single column output line upon which the images stored in the plurality of storage nodes may be read out. One or more row decoding circuits may be connected to the row select lines to select a row of one of the stored images within the storage nodes.
In third and fourth embodiments of an active pixel according to the present invention, one of which provides a current-mode output, the active pixel sensor includes a plurality of storage nodes, a plurality of image select signals for selecting the plurality of storage nodes, and a single row select line for placing selected images on a column output line. The use of the image select signals in combination with the row select transistor eliminates the need for the multiple row select signals of the third embodiment.
In a fifth embodiment of an active pixel according to the present invention, the active pixel sensor includes multiple storage nodes, multiple row select lines, and multiple column output lines.